In an air handling unit, outside or ventilation air and return air from the conditioned space are mixed, conditioned, and blown out of the air handling unit to the conditioned space. Typically, the outside and return air streams are mixed in a separate mixing box before entering the air handling unit. Once inside the air handling unit, the mixed streams of air pass through a filter, and then are conditioned by a plurality of heating or cooling coils. Once conditioned, the air is supplied to a conditioned space by a blower.
Unfortunately, when incoming air becomes stratified, or separated into layers having different temperatures, sub-freezing layers of air can freeze the conditioning coil, causing damage to the air handling unit. The likelihood of stratification increases in proportion to a temperature difference between the outside and return air streams. Stratification may cause nuisance trips of low temperature safety thermostats or incorrect control of air distribution when sensors cannot read the true mix temperature of the air stream, but most detrimentally, a combination of poor mixing and sub-freezing air streams can lead to freeze-up of a part of the chilled or hot water coils. Coil freeze-up greatly reduces effective heating or cooling capacity of the coil and, in serious cases, can lead to ruptured coils that require expensive repair or replacement. As one would expect, nuisance trips and coil freeze-ups are common during extreme cold weather conditions.
To prevent coil freeze-up, the installation of preheat coils in condensate traps, along with relocation of the coil, have been used help reduce the chances of coil freeze-up. Or, since stratification of the incoming air streams is believed to be a principal cause of nuisance trips and coil freeze-up, some known devices have attempted to reduce stratification by improved air mixing. Unfortunately, these solutions do not completely eliminate coil freeze-ups.
U.S. Pat. No. 5,031,515 describes a method for regulating outside air. This patent incorporates a conventional mixing chamber having an outside air damper and a room air damper. The patent adds an additional or relocated outlet air damper to improve control of the ratio of outside and return air within the total airflow. While this system is useful in obtaining desired ratios of return and outside air, it, unfortunately, does not provide instruction as to how to reduce air stratification.
U.S. Pat. No. 5,324,229 describes an air handling unit containing two fans and four sets of dampers. By controlling the damper position and air handler geometry, this patent improves mixing and reduces stratification by increasing the velocity of the cold outside air streams. Unfortunately, this patent requires replacement of existing air handling units to reduce the effects of stratification.
To further prevent coil freeze-up, known systems measure temperatures at several points within an air handling unit. Some known systems attempt to prevent coil freeze-up by installing sensors on or near the coil. But, due to large coil surface area, it is difficult to pinpoint the coldest location for a given set of operating conditions. Systems having sensors on the coil face prevent freeze-up by shutting off the air handling unit when an undesirable coil face temperature is reached. Disadvantageously, this form of control generally requires human oversight to reset the controller and/or to make sure that favorable conditions exist before turning the air handling unit on.
Consequently, there is a need for a method of ensuring that sub-freezing conditions are avoided at the coil face, without the need for sensors on the coil face while an air handling unit is in operation. There is a further need for a method of preventing freeze-up that utilizes fewer sensors and/or eliminates reliance on low-limit or shutdown sensor technology. Still a further need exists for a method of reducing stratification that does not require air handling unit replacement.